1. Field of the Invention
The present invention relates to high-frequency signal lines and electronic devices, more particularly to a high-frequency signal line including a signal line provided on a flexible element assembly, and an electronic device including the high-frequency signal line.
2. Description of the Related Art
In the case of a high-frequency signal line having a tri-plate stripline structure formed by a signal line being positioned between upper and lower ground conductors, the signal line is widened in order to reduce high-frequency transmission loss. Accordingly, the signal line has an increased surface area, and the ground conductors have larger areas opposed to the signal line. As a result, high-frequency transmission loss in the signal line is reduced. The high-frequency transmission loss refers to a loss mainly caused by a high-frequency signal being converted into heat in an impedance-matching situation.
However, widening the signal line increases the area of the signal line that is opposed to the ground conductors, resulting in increased capacitance between the signal line and the ground conductors. Accordingly, to set the characteristic impedance of the high-frequency signal line at a predetermined characteristic impedance value (e.g., 50Ω), it is necessary to increase the distance between the signal line and each ground conductor, thereby reducing the capacitance therebetween. However, increasing the distance between the signal line and the ground conductor results in an increased thickness of the high-frequency signal line.
Accordingly, there is a flexible board proposed by Japanese Patent Laid-Open Publication No. 2007-123740. FIG. 12 illustrates the flexible board 600 disclosed in Japanese Patent Laid-Open Publication No. 2007-123740, as viewed in a plan view in the direction of lamination.
The flexible board 600 includes a signal line 602 and a ground layer 604. The signal line 602 is a linear conductor. The ground layer 604 is laminated on a dielectric layer provided above the signal line 602 in the direction of lamination. Moreover, although not shown, another ground layer is provided below the signal line 602 in the direction of lamination. In addition, the flexible board 600 has a plurality of openings 606 provided in the ground layer 604. The openings 606 are in the form of rectangles aligned above the signal line 602 in the direction in which the signal line 602 extends. As a result, the signal line 602, when viewed in a top view in the direction of lamination, overlaps in part with the ground layer 604. Therefore, the capacitance that is created between the signal line 602 and the ground layer 604 is reduced. Thus, the distance between the signal line 602 and the ground layer 604 can be reduced, so that the flexible board 600 can be rendered thinner.
In this manner, various attempts have been made to achieve thinner high-frequency signal lines.